The present invention relates to a method of displaying alphanumerical and graphic characters, and further relates to a terminal for use of this method.
Cathode-ray tubes which are driven in a suitable manner are usually used for the display or "visualisation" of alphanumerical and graphic output information of digital processing systems.
When the cathode-ray tube is connected to suitable logic circuits, it is possible to control the number of characters per written line as well as the number of horizontal lines forming each character, the number of written lines, the generation of characters which correspond to the character code, the memory storage of the character positions on the written page, the movement and positioning of a cursor, writing-in by means of a keyboard etc. so that terminals of different types are obtained ("active", "passive", "intelligent").
The system which is at present predominantly used for displaying characters on a screen is based on a scanning method which corresponds to the scanning method used in television. On the luminescent material (the so-called "phosphor") of the cathode-ray tube a number of defined dots are excited and light up, a train of pulses being sequentially applied to the input of the tube, which pulses correspond to the pulses which are obtained by successively reading the individual dots of each horizontal line of the characters which form part of a written line.
In the operating conditions which are at present usually required for high-resolution monitors (2000-6000 characters) it must be taken into account that the individual dot on the screen is excited every 20 milliseconds for 20-30 nanoseconds, with a duty cycle of approximately 1.10.sup.-6.
The light emission caused thereby depends on the decay properties of the luminescent materials used.
To improve the legibility of characters, a great number of parameters must be processed, such as the dimensions and shape of each character, contrast, colour, electrooptical aberrations, brightness, etc. When it is desired to increase the brightness of the character, the current of the impinging electron beam must be increased; however, this increase causes a larger diameter of the spot, as a result of which the resolution becomes poorer, and also causes a decrease in contrast due to an increase of the halo across the thickness of the luminescent material.